Twitter was forced to roll back its software to a previous stable version on Thursday in order to restore service after a two-hour outage, one of its engineers said in a blog post.

Neither hackers nor animated GIFs for user avatars caused the service to crash around 9 a.m. Pacific Time, wrote Mazen Rawashdeh, vice president of engineering.

At least one hacking group claimed responsibility, saying it took Twitter down with a distributed denial-of-service (DDOS) attack, but Twitter says hackers were not the cause. An office move by Twitter and tweets related to the Euro 2012 soccer tournament also were not factors, Rawashdeh wrote.

He reiterated Twitter's message from earlier in the day that blamed a "cascading bug," which Rawashdeh described as a bug that isn't "confined to a particular software element, but rather its effect 'cascades' into other elements as well."

"One of the characteristics of such a bug is that it can have a significant impact on all users, worldwide, which was the case today," Rawashdeh wrote. "As soon as we discovered it, we took corrective actions, which included rolling back to a previous stable version of Twitter."

Twitter came back online briefly around 10:10 a.m. Pacific Time but failed a half-hour later. The site recovered by 11:08 a.m. Pacific Time, Rawashdeh wrote, and a review into the cause is under way.

"It's imperative that we remain available around the world, and today we stumbled," Rawashdeh wrote. "For that we offer our most sincere apologies and hope you'll be able to breathe easier now."

Rawashdeh wrote that Twitter has had some of its highest average marks for reliability and stability over the past six months, experiencing as little as 20 seconds of downtime in a 24-hour period.